Methotrexate (MTX) an antimetabolite of folic acid (FA), administered at low doses for long-term periods is one of the most efficacious treatment for a number of autoimmune diseases, including rheumatoid arthritis (RA). Toxicity, including nausea, vomiting, stomatitis, and cytopenias limit its clinical usefulness. The mechanism of toxicity is not known. Previous studies by this applicant, at this institution, indicate that FA supplementation (1 mg/day) ameliorates the toxicity of MTX in RA. We hypothesize that there is an optimal ratio of MTX to folate in the treatment of RA, which will allow toxicity to be minimized with no loss of efficacy. Further, the optimal ratio may be biochemically monitored. We propose to enroll ninety-three patients with Ra into a double-blind, placebo-controlled trial to evaluate the effect of 2 levels of FA supplementation in lessening toxicity while preserving efficacy. Simultaneous determinations of MTX and folate levels in plasma and peripheral blood mononuclear cells (PBMCs) and homocysteine levels in plasma, will be done to monitor the course of therapy. The precise mechanism of action of MTX in RA and other autoimmune diseases is not known. We hypothesize that it depends upon the inhibition of some but not all of the enzyme inhibitory activities of this drug. Using the MRL/1pr murine model of system lupus erythematosus, antifolates with narrower enzymatic inhibition patterns than MTX (eg selective inhibitors of the biosynthesis of thymidylate or purine nucleotides) will be tested. The MRL/1pr mouse model is an excellent model of autoimmune disease with manifestations including lymphoproliferation, glomerulonephritis, arthritis, vasculitic skin sores, and alopecia. Extent of hind limb pathology, abatement of proteinuria, severity of lymphoproliferation, reduction of vasculitic skin lesions, and survival will be markers of efficacy of the antifolate. Alterations in growth rate and early death will be markers of toxicity. Comparisons across antifolates should determine which folate-dependent pathways must be inhibited to produce efficacy with minimal toxicity. The animal studies will provide information to develop more selective drugs for use in human autoimmune disease. The protocol will be carried out at the University of Alabama at Birmingham, which as a Clinical Nutrition Research Unit, funded through the National Cancer Institute. These studies will be done in collaboration with faculty from the Division of Clinical Immunology and Rheumatology, which is the site of a Multipurpose Arthritis Center, and GCRC in the University Hospital.